Use of RF stability as basis to control configuration of MU-MIMO service

ABSTRACT

A method and system for controlling application of MU-MIMO. The disclosure provides for considering a device&#39;s rate of change of RF conditions as a basis to decide whether to provide the device with MU-MIMO service. For instance, a base station could determine which of the base station&#39;s served devices each have threshold low rate of change of RF conditions. And on at least that basis, the base station could select each such device to receive MU-MIMO service. Or faced with a choice between devices to receive MU-MIMO service, the base station could compare the devices&#39; rates of change of RF conditions and could select the devices that have lower rate of change of RF conditions to receive MU-MIMO service.

BACKGROUND

A wireless communication system typically includes a number of basestations that are configured to provide wireless coverage areas, such ascells and cell sectors, in which user equipment devices (UEs) such ascell phones, tablet computers, tracking devices, embedded wirelessmodules, and other wirelessly equipped communication devices (whether ornot user operated), can operate. In turn, each base station could becoupled with network infrastructure that provides connectivity with oneor more transport networks, such as the public switched telephonenetwork (PSTN) and/or the Internet for instance. With this arrangement,a UE within coverage of the system could engage in air interfacecommunication with a base station and could thereby communicate via thebase station with various remote network entities or with other UEsserved by the base station.

Each coverage area in such a system could operate in accordance with aparticular radio access technology, with air-interface communicationsfrom the base stations to UEs defining a downlink or forward link andair-interface communications from the UEs to the base stations definingan uplink or reverse link.

Over the years, the industry has embraced various “generations” of radioaccess technologies, in a continuous effort to increase available datarate and quality of service for end users. These generations have rangedfrom “1G,” which used simple analog frequency modulation to facilitatebasic voice-call service, to “4G”—such as Long Term Evolution (LTE),which facilitates mobile broadband service using technologies such asorthogonal frequency division multiplexing (OFDM) and multiple inputmultiple output (MIMO). And most recently, the industry is now exploringdevelopments in “5G” and particularly “5G NR” (5G New Radio), which mayuse a scalable OFDM air interface, advanced channel coding,massive-MIMO, beamforming, and/or other features, to support higher datarates and countless applications, such as mission-critical services,enhanced mobile broadband, and massive Internet of Things (IoT).

In accordance with the radio access technology, each coverage area couldoperate on a carrier, which could be frequency division duplex (FDD),defining separate frequency channels for downlink and uplinkcommunication, or time division duplex (TDD), with a single frequencychannel multiplexed over time between downlink and uplink use. Further,on the downlink and uplink, the carrier could be structured to definevarious physical channels for carrying information between the basestations and UEs. For example, the air interface could be divided overtime into frames, each divided in turn into subframes and timeslots, andthe carrier bandwidth could be divided over frequency into subcarriers,which could be grouped within each timeslot to define physical resourceblocks (PRBs) in which the subcarriers can be modulated to carry data.

The base station could then be configured to coordinate use of theseair-interface resources on an as-needed basis. For example, when thebase station has data to transmit to a UE, the base station couldallocate particular downlink air-interface resources to carry that dataand could accordingly transmit the data to the UE on the allocateddownlink resources. And when a UE has data to transmit to the basestation, the UE could transmit to the base station an uplink resourcegrant request, the base station could responsively allocate particularuplink air-interface resources to carry the data, and the UE could thentransmit the data to the base station on the allocated uplink resources.

OVERVIEW

One of the key performance metrics of a wireless communication system isits spectral efficiency, namely, the extent of data that the system cancarry per unit of frequency spectrum. The spectral efficiency of awireless communication system or its base stations could be measured asa quantity of bits per Hertz.

If a wireless communication system has low spectral efficiency, aprovider of the system may need to configure the system with additionallicensed spectrum, such as wider carriers and/or more carriers, in orderto accommodate subscriber communication needs. However, adding licensedspectrum could be costly and therefore undesirable.

One way to help improve spectral efficiency is to make use of MIMOtechnology.

With MIMO, a base station can engage in air interface communicationconcurrently on multiple different radio-frequency (RF) propagationpaths, or MIMO “layers,” with multiple layers occupying the samefrequency resources (e.g., subcarriers and PRBs) as each other. Tofacilitate this, the base station could be equipped with a MIMO antennaarray, comprising multiple transmit antennas and multiple receiveantennas. By suitably weighting and precoding transmissions byparticular antennas in the array, the base station can then outputspatially separate but concurrent transmissions for receipt by itsserved UEs. Because these concurrent transmissions occupy the samefrequency resources (e.g., subcarriers) as each other, MIMO can therebysupport a greater extent of data communication per unit frequency,thereby increasing the base stations' spectral efficiency and possiblyavoiding or deferring the need to add more spectrum.

MIMO service could be used in a “single-user MIMO” (SU-MIMO)configuration to increase the data rate of transmission to a single UE,by multiplexing communications to the UE onto multiple separate layersusing the same air-interface resources as each other. For instance, whena base station has data to transmit to a UE, the base station couldtime-division-multiplex the data into multiple data streams, the basestation could modulate the data streams onto the same PRBs as eachother, and the base station could output the modulated data streams ontoseparate antenna ports for concurrent transmission on separaterespective propagation paths to the UE. In practice, the UE could havetwo or more antennas, and the UE could estimate the channel distortionat each of its antennas and use the estimates to separately compute anduncover each of the base station's transmit signals.

Further, MIMO can also be used in a “multi-user MIMO” (MU-MIMO)configuration to increase the data capacity of the air interface byallowing communications to multiple UEs to use the same air-interfaceresources as each other. For instance, a base station can modulate datastreams destined to each of multiple UEs on the same PRBs as each otherand can transmit the modulated data streams on a separate respectivepropagation paths for receipt by the UEs. To facilitate this, the basestation could pre-code transmissions on each propagation path usingweighted coefficients based on channel estimates from the UEs, in amanner that enables each UE to remove cross-talk and receive itsintended data. Further, the base station could beamform thetransmissions respectively to each UE to help physically distinguish thetransmissions from each other. In theory, MU-MIMO could thus increasethe data capacity of the air interface by allowing a base station toserve more UEs at a time without requiring additional air-interfaceresources.

In dense urban markets and other areas, wireless service providers mayface a need to provide an increased extent of MIMO service. Inparticular, in such areas, a provider may serve a great many UEs or mayotherwise need to support high aggregate throughput, but the providermay have limited available spectrum, such as a limited number of PRBsper timeslot. To help overcome that limitation, the provider may equipits base stations with a massive-MIMO antenna array.

While a traditional MIMO antenna array may include on the order of 2 to8 antennas, a massive-MIMO antenna array would include many moreantennas, perhaps on the order of tens, hundreds, or even thousands ofantennas. For instance, a representative massive-MIMO antenna arraycould include 128 antennas, of which 64 might be configured as transmitantennas and the other 64 might be configured as receive antennas. Withthis arrangement, if 4 transmit antennas are used per layer (e.g., tofacilitate beamforming), the massive-MIMO antenna array might support onthe order of 16 layers, to facilitate concurrent transmissions to up to16 UEs (e.g., 8 UEs with 2 layers apiece, or 16 UEs with 1 layer apiece)or transmission to a single UE with up to 16 layers, among otherpossibilities.

When a base station serves many UEs at once, the base station couldbeneficially apply MU-MIMO in order to provide concurrenthigh-throughput transmissions to the UEs. For example, if the basestation's air interface has 100 PRBs per timeslot and the base stationhas a massive-MIMO array as discussed above, then, with MU-MIMO, thebase station could theoretically transmit concurrently to 8 UEs with 2layers apiece on all 100 of those PRBs. Within one timeslot, each UEcould thus theoretically receive two times the single-layer datacapacity of those 100 PRBs. (By comparison, if instead of applyingMU-MIMO, the base station were to apply just SU-MIMO with 2 layersapiece for each of those 8 UEs, then the base station might transmit toeach UE with 2 layers on just about 12 PRBs, thus providing lowerthroughput.)

To facilitate MU-MIMO service, the UEs that will share air-interfaceresources (e.g., PRBs) should be “orthogonal” to each other, meaningthat each UE could receive spatially separate transmissions from thebase station without undue interference from the base station'stransmissions to each other UE. Thus, when a base station is going toapply MU-MIMO (perhaps in response to the base station being heavilyloaded with connected UEs with high throughput requirements), the basestation could select a group of UEs to be a MU-MIMO group based on theUEs being orthogonal to each other. The base station could deem the UEsof a group to be sufficiently orthogonal to each other if each UE hasreported threshold high signal-to-noise-plus-interference ratio (SINR),and/or if the UEs are located at positions that are physically separateenough from each other that the base station's RF transmission paths tothe UEs would have sufficient angular separation, among otherpossibilities.

Further, given that the base station may support only up to a limitedquantity of MIMO layers (e.g., just 16 layers in the example above), thebase station may need to decide which UEs to provide with MU-MIMOservice. For instance, if the base station is going to provide just 8UEs with MU-MIMO service using 2 layers per UE, and if the base stationis serving well more than 8 UEs, the base station may need to decidewhich 8 UEs to provide with MU-MIMO service (in addition to deciding howto group the UEs, based on their orthogonality).

Optimally, when a base station selects UEs to receive MU-MIMO service,the base station could select the UEs in a manner that will help achieveone or more specific technological goals. For instance, the base stationcould select UEs based on a determination that the selected UEs wouldcontribute substantially to the base station's overall spectralefficiency—in an effort to avoid or defer the need to configure the basestation with costly additional spectrum. Further, the base station couldselect UEs based on a consideration of a determination that MU-MIMOservice would be especially helpful depending on the UEs' operationalstates.

In one respect, for example, the base station could select a UE toreceive MU-MIMO service based on the selected UE having threshold lowblock error rate (BLER) (e.g., not having threshold high BLER). BLERrepresents accuracy of data received by the UE, perhaps as a ratiobetween the number of erroneous transport blocks (e.g., with failedcyclic redundancy check) that the UE has received from the base stationand the total number of transport blocks that the UE has received fromthe base station. UEs that have low BLER could successfully receive moredata in a given quantity of air-interface resources, which could helpimprove the base station's overall spectral efficiency.

Thus, in an example implementation, the base station could determinewhich of the base station's served UEs each have threshold low BLER(e.g., do not have threshold high BLER), and the base station couldselect those UEs to receive MU-MIMO service. Or faced with a choicebetween UEs, the base station could compare the UEs' levels of BLER andcould select the UEs that have lower BLER to receive MU-MIMO service.

In another respect, the base station could select a UE to receiveMU-MIMO service based on the selected UE having low power headroom.Power headroom represents the difference between the UE's currently setuplink transmission power and the UE's maximum allowed uplinktransmission power. In practice, when a UE has low power headroom, thatcould mean that the UE's current uplink transmission power is high,which—for a battery powered UE—could mean that the UE's battery level islow or may soon become low. In that situation, it could be useful forthe base station to provide the UE with MU-MIMO service, since MU-MIMOservice could enable higher throughput downlink transmission to the UE,possibly allowing the UE to receive a given quantity of data morequickly and with less overall battery power consumption.

Thus, in an example implementation, the base station could determinewhich of the base station's served UEs each have threshold low powerheadroom and could select those UEs to receive MU-MIMO service. Or facedwith a choice between UEs, the base station could compare the UEs'levels of power headroom and could select the UEs that have lower powerheadroom to receive MU-MIMO service.

In yet another respect, if the base station is faced with a choicebetween UEs that are all threshold distant from the base station, thebase station could select a UE to receive MU-MIMO service based on theselected UE being a high power UE (HPUE) rather than a standard power UE(SPUE). An HPUE can transmit with higher maximum power than a standardpower UE and may use that higher maximum transmission power when locatedrelatively far from the base station. As between an HPUE and an SPUEthat are similarly situated, the HPUE may be more likely than the SPUEto engage in successful uplink communication to the base station. Thus,upon receipt of downlink transmission from the base station, the HPUEmay be more likely than the SPUE to successfully communicate a positiveacknowledgement to the base station, which could again contribute tohigher overall spectral efficiency.

Thus, in an example implementation, the base station could determinewhich of the base station's served UEs are HPUEs versus SPUEs and couldselect the UEs to receive MU-MIMO service based on the UEs being HPUEsrather than SPUEs. Further, the base station could apply this processspecifically in a scenario where the UEs at issue are each thresholddistant from the base station, such as where the UEs have relativelyhigh transmission signal delay, and/or threshold distant geographiclocation.

In still another respect, the base station could select a UE to receiveMU-MIMO service based on the selected UE being stationary rather thanmoving, or based on the UE being relatively stationary. If a UE ismoving and receiving MU-MIMO service, the base station may need toregularly adjust the direction of one or more RF beams on which the basestation transmits to the UE, and possible errors in those adjustmentscould give rise to failed communications, which would negatively impactspectral efficiency. Whereas, if the UE is stationary, the base stationmay be able to more reliably transmit to the UE, which could contributeto higher spectral efficiency.

Thus, in an example implementation, the base station could determinewhich of the base station's served UEs are stationary rather than movingand could select those UEs to receive MU-MIMO service. Or faced with achoice between UEs that are all moving, the base station could determinewhich UEs are moving the least (e.g., at the slowest speed) and couldselect those UEs to receive MU-MIMO service.

Further, in yet another respect, the base station could select a UE toreceive MU-MIMO service based on the selected UE having relativelystable RF conditions rather than relatively fluctuating RF conditions.Here, the analysis could be focused on the reported downlink RFconditions (e.g., reported reference signal receive power (RSRP),reported reference signal receive quality (RSRQ), and/or reportedchannel quality indicator (CQI)) and/or determined uplink RF conditions(e.g., uplink sounding reference signal (SRS) strength or quality)). Ifa UE has highly fluctuating RF conditions, then there could be greatuncertainty as to whether the UE will be able to successfully receivetransmissions from the base station and whether the UE will be able tosuccessfully acknowledge those transmissions, and therefore providingsuch a UE with MU-MIMO service may be less certain to help improvespectral efficiency. Whereas if a UE has relatively stable (andsufficiently high quality) RF conditions, then successful downlink anduplink communications with the UE may be more certain, and providing theUE with MU-MIMO service may be more certain to help improve spectralefficiency.

Thus, in an example implementation, the base station could determinewhich of the base station's served UEs have relatively stable RFconditions and could select those UEs to receive MU-MIMO service. Orfaced with a choice between UEs, the base station could compare thelevels of stability of the UEs' RF conditions and could select forMU-MIMO service the UEs that have most stable RF conditions.

These as well as other aspects, advantages, and alternatives will becomeapparent to those of ordinary skill in the art by reading the followingdetailed description, with reference where appropriate to theaccompanying drawings. Further, it should be understood that thedescriptions provided in this overview and below are intended toillustrate aspects by way of example only and not by way of limitation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a network arrangement in whichaspects of the present disclosure can be implemented.

FIG. 2 is a simplified diagram of an example massive-MIMO antenna arraythat could be implemented according to the disclosure.

FIG. 3 is a flow chart depicting operations that can be carried out inaccordance with the disclosure.

FIG. 4 is another flow chart depicting operations that can be carriedout in accordance with the disclosure.

FIG. 5 is another flow chart depicting operations that can be carriedout in accordance with the disclosure.

FIG. 6 is another flow chart depicting operations that can be carriedout in accordance with the disclosure.

FIG. 7 is another flow chart depicting operations that can be carriedout in accordance with the disclosure.

FIG. 8 is another flow chart depicting operations that can be carriedout in accordance with the disclosure.

FIG. 9 is another flow chart depicting operations that can be carriedout in accordance with the disclosure.

FIG. 10 is another flow chart depicting operations that can be carriedout in accordance with the disclosure.

FIG. 11 is another flow chart depicting operations that can be carriedout in accordance with the disclosure.

FIG. 12 is another flow chart depicting operations that can be carriedout in accordance with the disclosure.

FIG. 13 is a simplified block diagram of a base station operable inaccordance with the disclosure.

DETAILED DESCRIPTION

Referring to the drawings, as noted above, FIG. 1 is a simplified blockdiagram of an example wireless communication system in which variousdisclosed features can be implemented. It should be understood, however,that numerous variations from this and other disclosed arrangements andoperations are possible. For example, elements or operations could beadded, removed, combined, distributed, re-ordered, or otherwisemodified. In addition, operations described as being performed by one ormore entities could be implemented in various ways, such as by aprocessor executing instructions stored in non-transitory data storage,along with associated circuitry or other hardware, for instance.

As shown in FIG. 1, the example wireless communication system includes arepresentative base station 12 having an antenna array 14 through whichthe base station is configured to provide coverage 16 on one or morecarriers in one or more frequency bands. Shown operating within coverageof the base station are then a plurality of UEs 18, which could bedevices of the type discussed above, among other possibilities.

The base station could be a macro base station of the type configured toprovide a wide range of coverage, and the antenna array could be mountedon a tower or other tall structure. Alternatively, the base stationcould take other forms, such as a small cell base station, a repeater, afemtocell base station, or the like, which might be configured toprovide a smaller range of coverage. The base station could beconfigured to operate according to a 4G, 5G, or other radio accesstechnology. For instance, the base station could be an LTE evolvedNode-B (eNB) or a 5GNR gigabit Node-B (gNB), among other possibilities.

The base station is shown coupled with a core network 20, which could bean enhanced packet core (EPC) network, next generation core (NGC)network, or another network including components supporting anapplicable radio access technology and providing connectivity with atleast one transport network 22, such as the Internet.

In an example implementation as shown, the core network 20 includes aserving gateway (SGW) 24, a packet data network gateway (PGW) 26, amobility management entity (MME) 28, and a home subscriber server (HSS)30. In particular, the base station has an interface with the SGW, theSGW has an interface with the PGW, and the PGW provides connectivitywith the transport network. Further, the base station has an interfacewith the MME, and the MME has an interface with the SGW and the HSS.

With this arrangement, the SGW and PGW cooperatively provide user-planeconnectivity between the base station and the transport network, toenable a UE served by the base station to engage in communication on thetransport network. And the MME operates as a controller to carry outoperations such as coordinating UE attachment and setup of user-planebearers. Further, the HSS includes or has access to a data storecontaining UE capabilities and service profile data and can work withthe MME to facilitate UE authentication.

As discussed above, the air interface between the base station and UEswithin its coverage could be structured to define various air interfaceresources.

For instance, in the time domain, the air interface could define acontinuum of 10-millisecond (ms) frames, each divided into ten 1-mssubframes, and each subframe could be further divided into a number oftimeslots, each additionally divided into symbol time segments. And inthe frequency domain, the bandwidth of each carrier on which the basestation operates could be divided into subcarriers with specifiedsubcarrier spacing on the order of 15 to 240 kHz. With this arrangement,the air interface on each carrier would define an array of resourceelements each occupying a subcarrier and symbol time segment, and thebase station and UEs could communicate with each other throughmodulation of the subcarriers to carry data in those resource elements.Variations of this arrangement are possible as well.

Further, particular groupings of resource elements on the air interfacecould be grouped together to define the PRBs discussed above. In anexample implementation, each PRB could span one timeslot in the timedomain and a group of subcarriers in the frequency domain. Depending onthe carrier bandwidth, the air interface could thus support a certainnumber of such PRBs across the bandwidth of the carrier within eachtimeslot.

In addition, certain resource elements on the downlink and uplink couldbe reserved for particular control-channel or shared-channelcommunications.

For instance, on the downlink, certain resource elements per subframe(or per downlink subframe in TDD) could be reserved to define a downlinkcontrol region for carrying control signaling such as schedulingdirectives and acknowledgements from the base station to UEs. And otherresource elements per subframe could be reserved to define a sharedchannel in which PRBs could carry scheduled data communications from thebase station to UEs.

Further, in certain subframes, a group of resource elements centered onthe center frequency of each carrier could be reserved to carrysynchronization signals that UEs could detect as a way to discovercoverage of the base station on the carrier and to establish frametiming. And in certain subframes, a group of resource elements alsocentered on the center frequency of the carrier could be reserved todefine a broadcast-channel for carrying system information messages,such as master information block (MIB) and system information block(SIB) messages that WCDs could read to obtain operational parameterssuch as carrier bandwidth and other information. Further, certainresource elements distributed in a predefined pattern throughout thecarrier bandwidth per subframe could be reserved to carry referencesignals that UEs could measure as a basis to evaluate coverage strengthand quality and to provide channel estimates to facilitate precoding,beamforming, or the like.

On the uplink, on the other hand, certain resource elements per subframe(or per uplink subframe in TDD) could be reserved to define an uplinkcontrol region for carrying control signaling such as access requests,channel-quality reports, scheduling requests, and acknowledgements, fromUEs to the base station. And other resource elements per subframe couldbe reserved to define a shared channel in which PRBs could carryscheduled data communications from UEs to the base station. Further,still other resources on the uplink could be reserved for other purposesas well, such as for carrying uplink reference signals or the like.

In operation, when a UE enters into coverage of the base station on acarrier, the UE could detect the base station's synchronization signaland could then read the base station's MIB or the like to determine thecarrier's bandwidth. The UE could then engage in random access signalingand Radio Resource Control (RRC) configuration signaling with the basestation to connect with the base station on the carrier, thus puttingthe UE in an RRC-connected mode.

Once the UE is connected with the base station, the UE could thentransmit to the base station an attach request, which the base stationcould forward to the MME for processing. And after working with the HSSto authenticate the UE, the MME could coordinate setup for the UE of oneor more user-plane bearers between the base station and the PGW, toenable the UE to engage in communication on the transport network.Further, the base station could establish for the UE a context recordindicating operational state of the UE, and the base station couldreceive from the UE and/or the HSS (via the MME) a set of capabilitiesand profile data for the UE and could store that data in the contextrecord for reference while serving the UE.

The base station could then serve the UE with data communications.

For instance, when data arrives at the base station for transmission tothe UE, the base station could allocate one or more downlink PRBs in asubframe for use to transmit at least a portion of the data, defining atransport block, to the UE. The base station could then transmit to theUE in the control region of that subframe a Downlink Control Information(DCI) message that designates the PRBs, and the base station couldaccordingly transmit the transport block to the UE in those designatedPRBs.

For each such downlink transmission, the UE could then determine if theUE received transport block successfully. For instance, the transmissioncould carry a cyclic redundancy check (CRC) value computed based on thetransport block, and the UE could compute a CRC based on the receivedtransport block and determine whether its computed CRC matches thatcarried by the transmission. If the UE receives the transmission anddetermines that the CRC matches or otherwise that the UE received thetransport block successfully, then the UE could transmit to the basestation a positive acknowledgement (ACK) control message, and the basestation could then proceed with transmission of a next transport block(if any) to the UE. Whereas, if the UE did not receive the transmissionor determined that the CRC did not match and thus that there was anerror in the received transport block, then the UE could transmit to thebase station a negative acknowledgement (NACK), in response to which thebase station could attempt the transmission again.

On the other hand, when the UE has data to transmit to the base station(e.g., for transmission on the transport network), the UE could transmitto the base station a scheduling request that carries with it a bufferstatus report (BSR) indicating how much data the UE has buffered fortransmission. And in response the base station, could allocate one ormore uplink PRBs in an upcoming subframe for carrying a transport blockof that data from the UE and could transmit to the UE a DCI message thatdesignates those upcoming PRBs. The UE could then accordingly transmitthe transport block to the base station in the designated PRBs.

As with downlink transmission, for each transport block that the UEtransmits to the base station, the base station could determine if thetransport block arrived successfully, based on a CRC analysis forinstance. And if the base station received the transmissionsuccessfully, then the base station could transmit to the UE an ACK andcould schedule a next uplink transmission from the UE. Whereas, if thebase station did not receive the transmission successfully, then thebase station could transmit to the UE a NACK, and the UE could attemptretransmission to the base station.

While the base station is so serving a connected UE, the UE could alsotransmit various operational information to the base station to assistwith the base station's allocation of PRBs and other service of the UE.

For example, the UE could regularly evaluate the quality of its airinterface connection with the base station, such as based onreference-signal receive strength and/or quality (e.g., SINR) and couldperiodically transmit to the base station a channel-quality-indicator(CQI) value that represents the UE's determined level of channelquality. When the base station is going to allocate PRBs to carry datato or from the UE, the base station could then map the UE's most recentreported CQI to an applicable modulation and coding scheme (MCS) thatdefines how much error-correction-coding to include with thetransmission and what modulation scheme to use for modulating the dataonto resource elements. Based on this analysis and on the quantity ofPRBs to be allocated, the base station could thus determine what sizetransport block of data could be carried, and the base station couldallocate those PRBs for carrying that transport block.

As another example, the UE could transmit to the base station a powerheadroom report (PHR), indicating the UE's current power headroom, whichas noted above could be the difference between the UE's currently settransmission power and the UE's maximum transmission power. A UE's powerheadroom could vary over time, as a result of the UE adjusting itstransmission power. For instance, the UE and base station could engagein a power-control process to dynamically increase or decrease the UE'stransmission power based on the strength of UE transmissions received bythe base station. Thus, the UE's transmission power could vary from timeto time and the UE's power headroom could also vary accordingly.

The UE could transmit its PHR to the base station within a schedulingrequest that the UE sends to the base station to request allocation ofair interface resources for uplink transmission by the UE. And the basestation could use the UE's reported power headroom as a further basisfor PRB allocation, such as in deciding how many PRBs to allocate forthe UE per subframe.

In practice, the base station may serve multiple connected UEs at atime, and the base station may face a need to schedule datatransmissions concurrently to multiple such UEs and/or from multiplesuch UEs. Because the base station's air interface has just a finite,limited number of PRBs per unit time (e.g., per subframe), the basestation could implement a scheduling algorithm in an effort to fairlyand appropriately allocate the PRBs among the base station's served UEs.

Yet as noted above, the base station may still face load issues. Forinstance, there could be times when the base station is serving manyconnected UEs at once and faces a need to schedule data transmissions tomany such UEs at once, but the base station does not have sufficientPRBs per unit time to adequately meet the throughput needs of the UEs.

As noted above, the base station could use MIMO to help overcome thisproblem. In particular, the base station could apply MU-MIMO tofacilitate serving multiple UEs at once on the same PRBs as each otherand may thereby be able to provide the multiple UEs with a desired levelof throughput while also achieving improved spectral efficiency. Asfurther noted above, one way to provide such MIMO service is with use ofa massive-MIMO antenna array. Thus, in an example implementation, thebase station's antenna array 14 could be a massive-MIMO array.

FIG. 2 is a simplified diagram of an example massive-MIMO array thatcould be implemented at base station 12. In this illustration, each Xrepresents two antennas (or antenna elements), one with positivepolarization and one with negative polarization. As a result, eachcolumn in the example array includes eight antennas with positivepolarization and eight antennas with negative polarization. As there areeight columns of antennas, the massive-MIMO array thus has a total of128 antennas. In a representative implementation, 64 of these antennascould be configured as transmit (downlink) antennas, and the other 64could be configured as receive (uplink) antennas. For instance, all ofthe antennas with positive polarization could be configured as transmitantennas, and all of the antennas with negative polarization could beconfigured as receive antennas.

As discussed above, with this massive-MIMO antenna array, the basestation may be able to provide 16 layers of MIMO transmission. Forinstance, for each layer, the base station could use 4 of its transmitantennas to provide beamformed transmission defining a respective MIMOlayer. Thus, the base station could in theory transmit with up to 16layers on a common set of air interface resources (e.g., PRBs). OtherMIMO arrangements are possible as well.

As noted, the base station could transmit individual MIMO layers to UEs,by beamforming and/or pre-coding the transmissions. For example, thebase station could beamform transmissions to an individual UE byevaluating angle of arrival of uplink signals (e.g., an uplinksounding-reference-signal) from the UE or determining geolocation of theUE, and setting phase and amplitude of downlink transmission fromvarious antenna elements so as to direct the transmission toward the UE.Further, the base station could pre-code individual transmissions to aUE to help the UE distinguish those transmissions from others andextract the transmissions from a combination of received downlinksignals. For instance, the base station could transmit one or moredownlink modulation reference signals (DMRSs) that the UE can receiveand evaluate in order to establish and report channel estimates, and thebase station could use those channel estimates as a basis to pre-codetransmissions with weighted coefficients that enable the UE to receiveand uncover the transmissions.

Assuming sufficient orthogonality between UEs of a group, the basestation could thus transmit to the UEs of the group on the same PRBs aseach other, i.e., at the same time and on the same subcarriers as eachother. The transmissions to the UEs would occupy the same PRBs as eachother, but would be distinguished from each other through beamforming,pre-coding, and/or one or more other mechanisms.

Thus, in each downlink subframe, the base station could allocate a givenset of PRBs to each of multiple UEs of a MU-MIMO group and could providedownlink transmission on the allocated PRB(s) respectively to each UE ofthe MU-MIMO group, using one or more MIMO layers respectively for eachUE.

The base station could thus theoretically allocate all of the PRBs of asubframe to the MU-MIMO group, so that each UE of the MU-MIMO group canreceive data transmissions on one or more MIMO layers within all ofthose PRBs. Alternatively, the base station might allocate some of thePRBs of the subframe to a first MU-MIMO group of UEs and transmit tothose UEs with MIMO layers on those PRBs, and the base station mightallocate other PRBs of the subframe to a second MU-MIMO group of UEs andtransmit to those UEs with MIMO layers on those PRBs. Further, the basestation might allocate some PRBs of the subframe for use without MIMO orin other ways.

To configure MU-MIMO service for a UE, the base station may engage insignaling with the UE to obtain channel estimates and precodinginformation, and the base station may work with the UE through DCIsignaling to prepare the UE for receipt of beamformed and pre-codedtransmissions or the like.

As discussed above, at issue in this process could be which UEs the basestation should provide with MU-MIMO service, i.e., which UEs the basestation should include in MU-MIMO group(s) that the base station willestablish. The base station may face this issue when the base station isheavily loaded, such as with a threshold high number of connected UEs,and when the served UEs could benefit from MU-MIMO service. Given thatthe base station can support only up to a finite number of MIMO layersat once (e.g., only up to 16 MIMO layers in the example discussedabove), if the base station is serving many connected UEs that couldbenefit from MU-MIMO service, the base station may need to decide whichof those UEs to provide with MU-MIMO service. More particularly, as to arepresentative served UE, the base station may need to decide whether ornot to provide that UE with MU-MIMO service.

In practice, the base station could address this question per downlinksubframe. For instance, in anticipation of each downlink subframe, thebase station could determine for which of its connected-mode UEs thebase station has data buffered for downlink transmission, how soon thedata needs to be transmitted, and how many UEs are at issue. Further,the base station could consider its air-interface capacity in thesubframe, such as how many PRBs are available for the base station toallocate in the subframe. And based on these and/or other factors, thebase station could determine that applying MU-MIMO for transmission inthe subframe could be helpful, and the base station could then work todetermine which of the UEs should receive the MU-MIMO service.

The question of which UEs to provide with MU-MIMO service is related tobut separate from the question of how to group the UEs that the basestation will provide with MU-MIMO service. As noted above, grouping ofUEs for MU-MIMO service on the same shared PRBs could require the UEs tobe orthogonal to each other. But regardless of which MU-MIMO group a UEwould be included in, at issue here is whether the base station shouldprovide the UE with MU-MIMO service.

As noted above, the base station could take into account various factorsas a basis to decide which of the base station's served UEs to providewith MU-MIMO service, or to decide whether to provide a given served UEwith MU-MIMO service. Further, the base station could take into accountcombinations of such factors as a basis to make these decisions.

Selecting UEs for MU-MIMO Service Based on BLER

One factor that the base station could consider as a basis to decidewhether to provide a UE with MU-MIMO service is BLER with respect todata received by the UE, since low BLER represents more successful datatransmission per air-interface resource, which could contribute tohigher spectral efficiency. Thus, as noted above, the base station coulddetermine which of the base station's served UEs each have threshold lowBLER (e.g., which UEs do not have threshold high BLER). And on at leastthat basis, the base station could select those UEs to receive MU-MIMOservice. Or faced with a choice between UEs, the base station couldcompare the UEs' levels of BLER and could select the UEs that have lowerBLER to receive MU-MIMO service.

To enable the base station to best evaluate BLER experienced by itsserved UEs, each UE could be configured to track its own BLER and toreport its BLER to the base station or to another network entity (e.g.,an element management system) from which the base station could obtainthe UE's BLER data. For instance, each UE could track its BLER as a rateof erroneous transport blocks that the UE has received per unit time,possibly over a recent sliding window or otherwise rolled up over time.And the UE could report its BLER periodically or in response to one ormore other triggers, within a dedicated signaling message or in a fieldof another message. The base station could thus keep track of reportedBLER on a per-UE basis, perhaps recording each UE's latest reported BLERin a UE context record or the like.

When the base station is considering which of the base station's servedUEs should receive MU-MIMO service, the base station could then refer tothat reported BLER data to determine which UEs have BLER lower than apredefined threshold level, with the threshold being set by engineeringdesign or otherwise to represent a level where MU-MIMO service maycontribute adequately to spectral efficiency. If the base station thusdetermines that a UE's BLER is threshold low (e.g., not threshold high),then based on at least that factor, the base station could select the UEto receive MU-MIMO service. Whereas, if the base station determines thatthe UE's BLER is not threshold low (e.g., is threshold high), then,based on at least that factor, the base station could decide to notprovide that UE with MU-MIMO service.

Alternatively or additionally, the base station could perform acomparison between BLER of its various served UEs and could determinebased on that comparison whether a UE should receive MU-MIMO service orwhich UE(s) should receive MU-MIMO service. For instance, the basestation could compare the BLER of a first served UE with the BLER of asecond served UE. And based at least on a determination that the firstUE's BLER is lower (perhaps threshold lower) than the second UE's BLER,the base station could select the first UE rather than the second UE toreceive MU-MIMO service.

On the other hand, having all connected UEs regularly report their BLERto the base station could unfortunately bog down the air interface, aseach such report could itself consume uplink air interface resources. Toavoid this problem, the base station could direct some or all of itsserved UEs to report their BLER to the base station only when their BLERis threshold high, and the base station could then exclude fromconsideration for receiving MU-MIMO service any UE that has recentlyenough reported that it has a threshold high BLER. The base stationcould then assume that each UE that has not recently enough reportedhaving threshold high BLER does not have threshold high BLER (e.g., hasthreshold low BLER), and on at least that basis the base station couldselect each such UE to receive MU-MIMO service.

The base station could direct some or all of its UEs in various ways toreport threshold-high BLER. For instance, the base station's served UEsmight normally operate in a mode in which they do not report their BLERto the base station. But the base station could broadcast a systemmessage to which its served UEs will respond by transitioning to a modein which they will report their BLER to the base station when their BLERbecomes threshold high, i.e., responsive to their BLER becomingthreshold high. For example, the base station could include in a SIBmessage a bit or other value to which the UEs are configured to respondby transitioning to that mode. Further, the base station could specifyin the broadcast message a BLER threshold that the UEs are to considerfor this purpose, or the UEs could be configured in advance with anindication of that threshold.

Alternatively, the base station could configure individual UEs toprovide such threshold-high-BLER reports. For instance, the base stationcould transmit to each such UE an RRC connection reconfiguration messageor other unicast message that directs the UE to report its BLER when theBLER becomes threshold high, and the base station could also specify inthat message an applicable BLER threshold.

Given that MU-MIMO would be especially useful when the base station isheavily loaded, this reconfiguration of the base station's served UEs tooperate in a mode when they would report threshold-high BLER could bedone in response to the base station being heavily loaded. For instance,the base station could detect that the base station is heavily loaded(e.g., serving a threshold great number of UEs, having high PRBoccupancy, or otherwise as discussed above). And in response, the basestation could then direct its served UEs to report to the base stationif and when they have threshold high BLER.

Further, the act of a UE reporting to the base station that the UE hasthreshold high BLER could involve the UE reporting its BLER as an indexvalue or other BLER value and the base station deeming that reportedBLER to be threshold high. Alternatively, if the UE will report onlywhen the UE has threshold high BLER, then the UE's report could moresimply be a Boolean flag or other value indicative of the UE havingthreshold high BLER.

FIG. 3 is a flow chart depicting operations that can be carried out inaccordance with this disclosure, to control transmission over an airinterface in a wireless communication system. As shown in FIG. 3, atblock 34, the operations includes a base station serving a plurality ofUEs over the air interface, where each UE of the plurality receivesrespective blocks of data transmitted from the base station to the UEand each UE has a respective BLER representing a rate at which theblocks of data received by the UE have been in error. At block 36, theoperations include the base station selecting at least one of the UEs ofthe plurality to serve with MU-MIMO over the air interface, with theselecting being based at least on the BLER respectively of each selectedUE. And at block 38, the operations include, based on the selecting, thebase station configuring MU-MIMO service of each selected UE whileserving any non-selected UE (e.g., each other UE) of the plurality ofUEs without use of MU-MIMO.

In line with the discussion above, the operations could additionallyinclude receiving respectively from each UE of the plurality a report ofthe BLER of the UE, such as receiving the report directly from the UE orreceiving the report from a network entity to which the UE reported theBLER of the UE. Further, the reported BLER of each UE could be based onfailed a CRC of one or more of the blocks of data received by the UE.

In addition, as discussed above, the act of selecting at least one ofthe UEs of the plurality to serve with MU-MIMO over the air interfacewith the selecting being based at least on the BLER respectively of eachselected UE could involve, for each UE of the plurality (i) making adetermination of whether the BLER of the UE is lower than a predefinedthreshold level, (ii) if the determination is that the BLER of the UE islower than the predefined threshold level, then, based at least on thedetermination, selecting the UE to serve with MU-MIMO over the airinterface, and (iii) if the determination is that the BLER of the UE isnot lower than the predefined level threshold, then, based at least onthe determination, not selecting the UE to serve with MU-MIMO over theair interface.

Further, the act of selecting at least one of the UEs of the pluralityto serve with MU-MIMO over the air interface with the selecting beingbased at least on the BLER respectively of each selected UE couldinvolve (i) comparing the BLER of a first one of the UEs of theplurality with the BLER of a second one of the UEs of the plurality,(ii) based on the comparing, determining that the BLER of the first UEis lower than the BLER of the second UE, and (iii) based on thedetermining, selecting the first UE rather than the second UE to receiveMU-MIMO service over the air interface.

Still additionally, as discussed above, the operations could include thebase station directing the UEs of the plurality to operate in a mode inwhich each UE of the plurality will report the UE's BLER to the basestation if and when the BLER of the UE is threshold high, and after sodirecting the UEs, the base station treating each UE of the pluralitythat does not report threshold high BLER as being a UE that hasthreshold low BLER, for purpose of deciding whether to select the UE toserve with MU-IMO over the air interface.

For instance, the base station could broadcast a directive to which eachUE of the plurality is configured to respond by transitioning from notoperating in the mode to operating in the mode and/or unicasting to eachUE of the plurality a directive to which the UE is configured to respondby transitioning from not operating in the mode to operating in themode. Further, the directing could be done in response to determiningthat the base station is heavily loaded.

Yet additionally, as discussed above, the operations could include thebase station including each selected UE in a MU-MIMO group of UEs basedon a further determination that the selected UE is orthogonal to eachother UE of the MU-MIMO group.

And still further, as discussed above, the act of the base stationconfiguring MU-MIMO service of each selected UE could involve the basestation causing air interface transmission between the base station andthe selected UE to occupy same time-frequency air interface resources(e.g., PRBs) as air interface transmission between the base station andanother UE of plurality. Whereas, the act of the base station servingeach other UE of the plurality of UEs without use of MU-MIMO couldinvolve the base station causing air interface transmission between thebase station and each other UE to occupy different time-frequency airinterface resources than air interface transmission between the basestation and any other UE of the plurality.

FIG. 4 is another flow chart depicting operations that can be carriedout in accordance with this disclosure, to control transmission over anair interface in a wireless communication system. As shown in FIG. 4, atblock 40, the operations include a base station serving a plurality ofUEs over the air interface, where each UE of the plurality receivesrespective blocks of data transmitted from the base station to the UEand each UE has a respective BLER representing a rate at which theblocks of data received by the UE have been in error. At block 42, theoperations include the base station determining, respectively for eachUE of the plurality, whether the BLER of the UE is threshold low. And atblock 44, the operations include the base station limiting applicationof MU-MIMO to UEs of the plurality whose BLER the base stationdetermined to be threshold low.

In line with the discussion above, the operations could additionallyinclude the base station causing each UE of the plurality to operate ina mode in which the UE will report when BLER of the UE is thresholdhigh. For instance, the base station could do this in response to adetermination the base station is threshold loaded. And the act ofdetermining for a UE of the plurality that BLER of the UE is thresholdlow could involve determining that the base station has not receivedfrom the UE a report indicating that the BLER of the UE is thresholdhigh.

Further, as discussed above, the act of the base station limitingapplication of MU-MIMO to UEs of the plurality whose BLER the basestation determined to be threshold low could involve (i) the basestation selecting at least a first UE of the plurality to receiveMU-MIMO service, where the selecting is based at least on the BLER ofthe first UE being at least as low as a predefined threshold level and(ii) the base station rejecting application of MU-MIMO service for atleast a second UE of the plurality, i.e., excluding the second UE fromreceiving MU-MIMO service, where the rejecting is based at least on theBLER of the second UE not being at least as low as the predefinedthreshold level.

Selecting UEs for MU-MIMO Service Based on Power Headroom

Another factor that the base station could consider as a basis to decidewhether to provide a UE with MU-MIMO service is power headroom of theUE, since low power headroom could suggest that the UE is operating withrelatively high transmission power, which could in turn suggest that theUE's battery level (if applicable) is being depleted more than it mightotherwise be.

As discussed above, providing a UE with MU-MIMO service might help theUE to complete downlink transmission quicker than it otherwise would,which might be useful when a UE is faced with possibly low or relativelyquickly-depleting battery energy. Thus, as noted above, the base stationcould determine which of the base station's served UEs each havethreshold low power headroom (e.g., which UEs do not have threshold highpower headroom). And on at least that basis, the base station couldselect those UEs to receive MU-MIMO service. Or faced with a choicebetween UEs, the base station could compare the UEs' levels of powerheadroom, and the base station could select the UEs that have lowerpower headroom to receive MU-MIMO service.

To enable the base station to use a UE's power headroom as a basis todecide whether to provide the UE with MU-MIMO service, the base stationcould keep track of the UE's latest reported power headroom. Forinstance, when the UE reports provides the base station with a PHRindicating the UE's power headroom, the base station could record thatpower headroom in a UE context record or the like.

When the base station is considering which UEs should receive MU-MIMOservice, the base station could refer to that reported power-headroomdata to determine which UEs have power headroom lower than a predefinedthreshold level, with the threshold being set by engineering design orotherwise as a level that may suggest likely fast battery-energydepletion. If the base station thus determines that a UE's powerheadroom is threshold low (e.g., not threshold high), then based on atleast that factor, the base station could select the UE to receiveMU-MIMO service. Whereas, if the base station determines that the UE'spower headroom is not threshold low (e.g., is threshold high), then,based on at least that factor, the base station could decide to notprovide that UE with MU-MIMO service.

Alternatively or additionally, the base station could perform acomparison between power headroom of its various served UEs and coulddetermine based on that comparison whether a UE should receive MU-MIMOservice or which UE(s) should receive MU-MIMO service. For instance, thebase station could compare the power headroom of a first served UE withthe power headroom of a second served UE. And based at least on adetermination that the first UE's power headroom is lower (perhapsthreshold lower) than the second UE's power headroom, the base stationcould select the first UE rather than the second UE to receive MU-MIMOservice.

FIG. 5 is a flow chart depicting operations that can be carried out inaccordance with this disclosure, to control transmission over an airinterface in a wireless communication system. As shown in FIG. 5, atblock 50, the operations includes a base station serving a plurality ofUEs over the air interface, where each UE of the plurality has arespective power headroom representing a difference between a maximumtransmission power of the UE and a current transmission power of the UE.At block 52, the operations include the base station selecting at leastone of the UEs of the plurality to serve with MU-MIMO over the airinterface, with the selecting being based at least on the power headroomrespectively of each selected UE. And at block 54, the operationsinclude, based on the selecting, the base station configuring MU-MIMOservice of each selected UE while serving any non-selected UE (e.g.,each other UE) of the plurality of UEs without use of MU-MIMO.

In line with the discussion above, the operations could additionallyinclude receiving respectively from each UE the plurality a report ofthe power headroom of the UE. For instance, the base station couldreceive such a report within a scheduling request from the UE.

In addition, as discussed above, the act of selecting at least one ofthe UEs of the plurality to serve with MU-MIMO over the air interfacewith the selecting being based at least on the power headroomrespectively of each selected UE could involve, for each UE of theplurality (i) making a determination of whether the power headroom ofthe UE is lower than a predefined threshold level, (ii) if thedetermination is that the power headroom of the UE is lower than thepredefined threshold level, then, based at least on the determination,selecting the UE to serve with MU-MIMO over the air interface, and (iii)if the determination is that the power headroom of the UE is not lowerthan the predefined level threshold, then, based at least on thedetermination, not selecting the UE to serve with MU-MIMO over the airinterface.

Further, the act of selecting at least one of the UEs of the pluralityto serve with MU-MIMO over the air interface with the selecting beingbased at least on the power headroom respectively of each selected UEcould involve (i) comparing the power headroom of a first one of the UEsof the plurality with the power headroom of a second one of the UEs ofthe plurality, (ii) based on the comparing, determining that the powerheadroom of the first UE is lower than the power headroom of the secondUE, and (iii) based on the determining, selecting the first UE ratherthan the second UE to receive MU-MIMO service over the air interface.

Still additionally, as discussed above, the operations could compriseincluding each selected UE in a MU-MIMO group of UEs based on a furtherdetermination that the selected UE is orthogonal to each other UE of theMU-MIMO group.

And still further, as discussed above, the act of the base stationconfiguring MU-MIMO service of each selected UE could involve the basestation causing air interface transmission between the base station andthe selected UE to occupy same time-frequency air interface resources(e.g., PRBs) as air interface transmission between the base station andanother UE of plurality. Whereas, the act of the base station servingeach other UE of the plurality of UEs without use of MU-MIMO couldinvolve the base station causing air interface transmission between thebase station and each other UE to occupy different time-frequency airinterface resources than air interface transmission between the basestation and any other UE of the plurality.

FIG. 6 is another flow chart depicting operations that can be carriedout in accordance with this disclosure, to control transmission over anair interface in a wireless communication system. As shown in FIG. 6, atblock 60, the operations include a base station serving a plurality ofUEs over the air interface, where each UE of the plurality has arespective power headroom representing a difference between a maximumtransmission power of the UE and a current transmission power of the UE.At block 62, the operations include the base station determining,respectively for each UE of the plurality, whether the power headroom ofthe UE is threshold low. And at block 64, the operations include thebase station limiting application of MU-MIMO to UEs of the pluralitywhose power headroom the base station determined to be threshold low.

In line with the discussion above, the operations could additionallyinclude receiving respectively from each UE the plurality a report ofthe power headroom of the UE. Here again, for instance, the base stationcould receive such a report within a scheduling request from the UE. Andthe operations could include the base station grouping UEs of theplurality into a MU-MIMO group based on a determination that the UEs ofthe MU-MIMO group are orthogonal to each other.

Further, the act of the base station limiting application of MU-MIMO toUEs of the plurality whose power headroom the base station determined tobe threshold low could involve (i) the base station selecting at least afirst UE of the plurality to receive MU-MIMO service, wherein theselecting is based at least on the power headroom of the first UE beingat least as low as a predefined threshold level, and (ii) rejecting bythe base station application of MU-MIMO service for at least a second UEof the plurality, wherein the rejecting is based at least on the powerheadroom of the second UE not being at least as low as the predefinedthreshold level.

Selecting UEs for MU-MIMO Service Based on Power Class

Another factor that the base station could consider as a basis to decidewhether to provide a UE with MU-MIMO service is power class of the UE,such as whether the UE is an HPUE or rather an SPUE, since UEs that cantransmit with higher power may be better able to successfullyacknowledge transmissions from the base station, which could contributeto higher spectral efficiency.

Industry standards or governmental regulations may define UE powerclasses, and UEs could be characterized by their manufacturingspecifications or other data to be a member of one power class oranother. For instance, standards may define SPUEs as UEs that arelimited to operating with up to a maximum transmission power of 23decibel-milliwatts (dBm) (about 0.2 Watts) and HPUEs as UEs that arelimited to operating with up to a maximum transmission power of 26 dBm(about 0.4 Watts) or more on certain carrier frequencies. With theability to operate at up to a higher maximum transmission power, an HPUEmay therefore be better able than an SPUE to transmit successfully tothe base station from a distance or with RF obstructions.

Thus, as noted above, the base station could determine which of the basestation's served UEs have a high power class rather than a low powerclass, such as which of the base station's served UEs are HPUEs ratherthan SPUEs. And based on at least that factor, the base station couldselect those UEs to receive MU-MIMO service. Or faced with a choicebetween UEs, the base station could compare the UEs' power classes andcould select the UEs that have a higher power class to receive MU-MIMOservice.

For instance, the base station could determine whether a UE is a HPUE oris rather an SPUE. And if the base station determines that the UE is anHPUE rather than an SPUE, then, based on at least that determination,the base station could decide to provide the UE with MU-MIMO service.Whereas, if the base station determines that the UE is an SPUE ratherthan an HPUE, then, based on at that determination, the base stationcould decide to not provide the UE with MU-MIMO service.

Further, because UEs are most likely to use their maximum transmissionpower when the UEs are located far away from the base station, the basestation's consideration of UE power class as a basis to decide whetherto provide the UE with MU-MIMO service could be specifically for UEsthat are located at least a threshold far away from the base station.For instance, the base station could determine which of its served UEsare located at least a predefined threshold distance from the basestation, and the base station could consider power classes of thosedetermined UEs, as a basis to determine which if any of those UEs toprovide with MU-MIMO service.

In an example implementation, the base station could determine the powerclass of a served UE by referring to the UE's profile (e.g.,capabilities) data, as obtained from the HSS and/or from the UE duringattachment or at another time. That data could specify the UE's powerclass in a manner interpretable and understandable by the base station,at least for the present purpose. For instance, if the UE is an SPUE,the data could specify the UE's power class by a first value, whereas ifthe UE is an HPUE, the data could specify the UE's power class by asecond value.

Further, the base station could determine the distance of a served UEfrom the base station in various ways. For instance, the base stationcould estimate the UE's distance based on evaluation of signal delay fortransmission between the UE and the base station. Alternatively, thebase station, the UE, and/or one or more other entities could determinethe UE's geographic location through trilateration, usingsatellite-positioning, or in another manner, and the base station couldcompare that location with the base station's geographic location, todetermine how far the UE is from the base station. Still alternatively,the base station could use UE reports of signal strength from one ormore neighboring base stations as a way to estimate the UE's locationgiven network coverage maps, and the base station could similarlycompute the UE's distance.

Thus, when the base station is considering which UEs should receiveMU-MIMO service, the base station could identify a subset (one or more)of the base station's served UEs based on their each being positioned atleast a predefined threshold distance from the base station, and thebase station could determine for each such UE whether the UE is an HPUEor rather an SPUE. If the base station thereby determines that athreshold distant UE is an HPUE rather than an SPUE, then, based atleast on that determination, the base station could decide to providethat distant UE with MU-MIMO service. Whereas, if the base stationthereby determines that a threshold distant UE is an SPUE rather than anHPUE, then, based at least on that determination, the base station coulddecide to not provide that distant UE with MU-MIMO service.

Alternatively or additionally, the base station could perform acomparison between power classes of various ones of its served UEs, suchas those UEs deemed to be threshold distant from the base station, andthe base station could decide based at least on that comparison whichUEs should receive MU-MIMO service and/or which UEs should not receiveMU-MIMO service. For instance, given first and second served UEs thatare located threshold far from the base station, the base station couldselect the first UE to receive MU-MIMO service based on the first UEhaving a higher power class (e.g., higher maximum transmission power)than the second UE, and the base station could decide to not serve thesecond UE with MU-MIMO based on the second UE having a lower power classthan the first UE.

FIG. 7 is a flow chart depicting operations that can be carried out inaccordance with this disclosure, to control transmission over an airinterface in a wireless communication system. As shown in FIG. 7, atblock 70, the operations includes a base station serving a plurality ofUEs over the air interface, where each UE of the plurality has arespective power class defining a maximum transmission power of the UE.At block 72, the operations include the base station selecting at leastone of the UEs of the plurality to serve with MU-MIMO over the airinterface, with the selecting being based at least on the power classrespectively of each selected UE. And at block 74, the operationsinclude, based on the selecting, the base station configuring MU-MIMOservice of each selected UE while serving any non-selected UE (e.g.,each other UE) of the plurality of UEs without use of MU-MIMO.

In line with the discussion above, the operations could additionallyinclude the base station determining, respectively for each UE theplurality, the power class of the UE. For instance, the base stationcould determine the power class of each UE by reference to profile dataof the UE.

Further, as discussed above, the power class of each UE could be astandard power class defining a first maximum transmission power or ahigh power class defining a second maximum transmission power higherthan the first maximum transmission power, among other possibilities.For instance, each UE could be an SPUE or an HPUE, among otherpossibilities. And the act of selecting at least one of the UEs based atleast on the power class respectively of each selected UE could involveselecting the UE based at least in part on the UE having the high powerclass (e.g., being an HPUE) rather than the standard power class (e.g.,being an SPUE).

As further discussed above, the act of selecting at least one of the UEsof the plurality to serve with MU-MIMO over the air interface with theselecting being based at least on the power class respectively of eachselected UE could then involve, for each UE of the plurality (i) makinga determination of whether the power class of the UE is lower than apredefined threshold level, (ii) if the determination is that the powerclass of the UE is the high power class rather than the low power class,then, based at least on the determination, selecting the UE to servewith MU-MIMO over the air interface, and (iii) if the determination isthat the power class of the UE is the standard power class rather thanthe high power class, then, based at least on the determination, notselecting the UE to serve with MU-MIMO over the air interface.

Further, the act of selecting at least one of the UEs of the pluralityto serve with MU-MIMO over the air interface with the selecting beingbased at least on the power class respectively of each selected UE couldinvolve (i) comparing the power class of a first one of the UEs of theplurality with the power class of a second one of the UEs of theplurality, (ii) based on the comparing, determining that the power classof the first UE is higher than the power class of the second UE, and(iii) based on the determining, selecting the first UE rather than thesecond UE to receive MU-MIMO service over the air interface.

Still additionally, as discussed above, this process could be focused onUEs that are threshold distant from the base station. Thus, theoperations could additionally include the base station initiallyidentifying the plurality of UEs based on each UE of the plurality ofUEs being located at least a predefined threshold distance from the basestation.

Further, the operations could comprise including each selected UE in aMU-MIMO group of UEs based on a further determination that the selectedUE is orthogonal to each other UE of the MU-MIMO group.

And still further, as discussed above, the act of the base stationconfiguring MU-MIMO service of each selected UE could involve the basestation causing air interface transmission between the base station andthe selected UE to occupy same time-frequency air interface resources(e.g., PRBs) as air interface transmission between the base station andanother UE of plurality. Whereas, the act of the base station servingeach other UE of the plurality of UEs without use of MU-MIMO couldinvolve the base station causing air interface transmission between thebase station and each other UE to occupy different time-frequency airinterface resources than air interface transmission between the basestation and any other UE of the plurality.

FIG. 8 is another flow chart depicting operations that can be carriedout in accordance with this disclosure, to control transmission over anair interface in a wireless communication system. As shown in FIG. 8, atblock 80, the operations include a base station serving a plurality ofUEs over the air interface, where each UE of the plurality has arespective power class defining a maximum transmission power of the UE.At block 82, the operations include the base station identifying asubset (one or more) of the served UEs based on each UE of the subsetbeing located at least a predefined threshold distance from the basestation.

At block 84, the operations include the base station determining,respectively for each UE of the subset, whether the power class of theUE is a high power class defining a first maximum transmission power orrather a low power class defining a second maximum transmission powerlower than the first maximum transmission power. And at block 86, theoperations include, as to the subset, the base station limitingapplication of MU-MIMO to each UE of the subset whose power class thebase station determined to be the high power class.

In line with the discussion above, the operations could additionallyinclude the base station determining, respectively for each UE of thesubset, the power class of the UE, such as by reference to profile dataof the UE.

Further, as discussed above, the act of the base station limitingapplication of MU-MIMO to each UE of the subset whose power class thebase station determined to be the high power class could involve, for agiven UE of the subset, (i) if the determined power class of the UE isthe high power class rather than the standard power class, then, basedat least on the determination, configuring MU-MIMO service of the UE and(ii) if the determination is that the power class of the UE is thestandard power class rather than the high power class, then, based atleast on the determination, not configuring MU-MIMO service of the UE.

And still further, the operations could include the base stationgrouping UEs of the plurality into a MU-MIMO group based on adetermination that the UEs of the MU-MIMO group are orthogonal to eachother.

Selecting UEs for MU-MIMO Service Based on Mobility

Another factor that the base station could consider as a basis to decidewhether to provide a UE with MU-MIMO service is whether the UE isstationary rather than moving, or whether the UE is sufficientlystationary. In particular, as noted above, the base station could selecta UE to receive MU-MIMO service based on the selected UE beingstationary rather than moving, or based on the UE being relativelystationary, as the stationary nature of the UE might make it easier forthe base station to more reliably beamform to the UE, which could helpimprove spectral efficiency.

Thus, the base station could determine which of the base station'sserved UEs are stationary rather than moving, or which of the basestation's served UEs are moving less than a predefined extent, and,based on at least that factor, could select those UEs to receive MU-MIMOservice. Or faced with a choice between UEs that are all moving, thebase station could determine which UEs are moving the least and, basedon at least that factor, could select those UEs to receive MU-MIMOservice.

To facilitate this analysis, the base station could determine in variousways the speed of movement of each of the base station's served UEs. Forinstance, the base station could repeatedly estimate the UE's distancefrom the base station based on evaluation of signal delay oftransmission between the UE and the base station, and the base stationcould track the rate of change of that distance as an indication of theUE's speed of movement. Alternatively, the base station, the UE, and/orone or more other entities could repeatedly determine the UE'sgeographic location through trilateration, using satellite-positioning,or in another manner, and the base station could track the rate ofchange of that position as an indication of the UE's speed of movement.Still alternatively, the base station could receive from the UE oranother entity an indication of the UE's speed of movement or couldotherwise determine the UE's speed of movement. Further, the basestation could record each UE's speed of movement in a UE context recordof the like.

When the base station is considering which of the base station's servedUEs should receive MU-MIMO service, the base station could then refer toits information about each UE's respective speed of movement, and thebase station could determine which of the UEs are moving less than apredefined threshold, with the threshold being set by engineering designor otherwise to represent a level where MU-MIMO service could bereliably established. If the base station determines that a UE's speedof movement is threshold low (e.g., not threshold high), then, based atleast on that factor, the base station could select the UE to receiveMU-MIMO service. Whereas, if the base station determines that the UE'sspeed of movement is not threshold low (e.g., is threshold high), then,based at least on that factor, the base station could decide to notprovide that UE with MU-MIMO service.

Alternatively or additionally, the base station could perform acomparison between speed of movement of its various served UEs and coulddetermine based on that comparison whether a UE should receive MU-MIMOservice or which UE(s) should receive MU-MIMO service. For instance, thebase station could compare the speed of movement of a first served UEwith the speed of movement of a second served UE. And based at least ona determination that the first UE's speed of movement is lower (perhapsthreshold lower) than the second UE's speed of movement, the basestation could select the first UE rather than the second UE to receiveMU-MIMO service.

FIG. 9 is a flow chart depicting operations that can be carried out inaccordance with this disclosure, to control transmission over an airinterface in a wireless communication system. As shown in FIG. 9, atblock 90, the operations includes a base station serving a plurality ofUEs over the air interface, where each UE of the plurality has arespective speed of movement (zero if stationary, or non-zero ifmoving). At block 92, the operations include the base station selectingat least one of the UEs of the plurality to serve with MU-MIMO over theair interface, with the selecting being based at least on the speed ofmovement respectively of each selected UE. And at block 94, theoperations include, based on the selecting, the base station configuringMU-MIMO service of each selected UE while serving any non-selected UE(e.g., each other UE) of the plurality of UEs without use of MU-MIMO.

In line with the discussion above, the operations could additionallyinclude determining (e.g., estimating) respectively for each UE theplurality the speed of movement of the UE. For instance, the basestation could determine the speed of movement of the UE based on changein signal delay of transmission between the UE and the base station orotherwise as discussed above.

In addition, as discussed above, the act of selecting at least one ofthe UEs of the plurality to serve with MU-MIMO over the air interfacewith the selecting being based at least on the speed of movementrespectively of each selected UE could involve, for each UE of theplurality (i) making a determination of whether the speed of movement ofthe UE is lower than a predefined threshold level, (ii) if thedetermination is that the speed of movement of the UE is lower than thepredefined threshold level, then, based at least on the determination,selecting the UE to serve with MU-MIMO over the air interface, and (iii)if the determination is that the speed of movement of the UE is notlower than the predefined level threshold, then, based at least on thedetermination, not selecting the UE to serve with MU-MIMO over the airinterface.

Further, the act of selecting at least one of the UEs of the pluralityto serve with MU-MIMO over the air interface with the selecting beingbased at least on the speed of movement respectively of each selected UEcould involve (i) comparing the speed of movement of a first one of theUEs of the plurality with the speed of movement of a second one of theUEs of the plurality, (ii) based on the comparing, determining that thespeed of movement of the first UE is lower than the speed of movement ofthe second UE, and (iii) based on the determining, selecting the firstUE rather than the second UE to receive MU-MIMO service over the airinterface.

Still additionally, as discussed above, the operations could compriseincluding each selected UE in a MU-MIMO group of UEs based on a furtherdetermination that the selected UE is orthogonal to each other UE of theMU-MIMO group.

And still further, as discussed above, the act of the base stationconfiguring MU-MIMO service of each selected UE could involve the basestation causing air interface transmission between the base station andthe selected UE to occupy same time-frequency air interface resources(e.g., PRBs) as air interface transmission between the base station andanother UE of plurality. Whereas, the act of the base station servingeach other UE of the plurality of UEs without use of MU-MIMO couldinvolve the base station causing air interface transmission between thebase station and each other UE to occupy different time-frequency airinterface resources than air interface transmission between the basestation and any other UE of the plurality.

FIG. 10 is another flow chart depicting operations that can be carriedout in accordance with this disclosure, to control transmission over anair interface in a wireless communication system. As shown in FIG. 10,at block 100, the operations include a base station serving a pluralityof UEs over the air interface, where each UE of the plurality has arespective speed of movement. At block 102, the operations include thebase station determining, respectively for each UE of the plurality,whether the speed of movement of the UE is threshold low. And at block104, the operations include the base station limiting application ofMU-MIMO to UEs of the plurality whose speed of movement the base stationdetermined to be threshold low.

In line with the discussion above, the operations could additionallyinclude determining respectively for each UE the plurality the speed ofmovement of the UE. Here again, for instance, the base station coulddetermine the UE's speed of movement in various ways as discussed above.

Further, the act of the base station limiting application of MU-MIMO toUEs of the plurality whose speed of movement the base station determinedto be threshold low could involve (i) the base station selecting atleast a first UE of the plurality to receive MU-MIMO service, whereinthe selecting is based at least on the speed of movement of the first UEbeing at least as low as a predefined threshold level, and (ii)rejecting by the base station application of MU-MIMO service for atleast a second UE of the plurality, wherein the rejecting is based atleast on the speed of movement of the second UE not being at least aslow as the predefined threshold level.

Selecting UEs for MU-MIMO Service Based on Stability of RF Conditions

Another factor that the base station could consider as a basis to decidewhether to provide a UE with MU-MIMO service is whether the UE's RFconditions are relatively stable rather than relatively fluctuating, orwhether the UE's RF conditions are sufficiently stable. In particular,as noted above, the base station could select a UE to receive MU-MIMOservice based on the selected UE having relatively stable RF conditionsrather than having relatively fluctuating RF conditions, as stable RFconditions could suggest increased certainty of successfulcommunications between the base station and the UE, which could helpincrease spectral efficiency.

Note that the term “RF conditions” could refer to one or more metricsrelated to RF communication between the base station and UE. Examples ofsuch metrics include CQI, SINR, RSRP, and RSRQ, BLER, and/orretransmission rate, among other possibilities.

Thus, the base station could determine which of the base station'sserved UEs have relatively stable RF conditions rather than relativelyfluctuating RF conditions, and, based on at least that factor, couldselect those UEs to receive MU-MIMO service. Or faced with a choicebetween UEs, the base station could determine which UEs have the moststable RF conditions and, based at least on that factor, the basestation could select those UEs to receive MU-MIMO service.

To facilitate this analysis, the base station could determine in variousways the level of stability of RF conditions respectively of each of thebase station's served UEs. For instance, the base station could gaugethe level of stability of the UE's RF conditions based on the rate ofchange of one or more metrics such as those noted above, with a higherrate of change corresponding with lower stability and vice versa.

In particular, the base station could determine a rate of changerespectively of one or more such metrics respectively for each UE of theplurality. Considering one such metric, the base station could determinea rate of change of the metric and could use that rate of change as abasis to determine whether to provide the UE with MU-MIMO service. Orconsidering multiple such metrics, the base station could determine arate of change respectively of each metric and could compute an average,weighted average, or other representation of a rate of change of thegroup of metrics, and the base station could consider that rate ofchange as a basis to determine whether to provide the UE with MU-MIMOservice.

For instance, for each UE, the base station could keep track of the UE'sreported CQI, RSRP, RSRQ, or SINR, and/or the base station could keeptrack of the base station's rate of retransmission to the UE (e.g.,responsive to NACKs from the UE) and/or one or more other metrics. Andthe base station could regularly compute for each UE a rate of change ofone or more such metrics, over a sliding window of time for instance.Further, the base station could store an associated indication of eachUE's rate of change of RF conditions in a UE context record or the like.

When the base station is considering which of the base station's servedUEs should receive MU-MIMO service, the base station could then refer toits information about each UE's respective rate of change of RFconditions, and the base station could determine which of the UEs have arate of change of RF conditions lower than a predefined threshold, withthe threshold being set by engineering design or otherwise to representa level where MU-MIMO service may contribute adequately to spectralefficiency. If the base station determines that a UE's rate of change ofRF conditions is threshold low (e.g., not threshold high), then, basedat least on that factor, the base station could select the UE to receiveMU-MIMO service. Whereas, if the base station determines that the UE'srate of change of RF conditions is not threshold low (e.g., is thresholdhigh), then, based at least on that factor, the base station coulddecide to not provide that UE with MU-MIMO service.

Alternatively or additionally, the base station could perform acomparison between rate of change of RF conditions of its various servedUEs and could determine based on that comparison whether a UE shouldreceive MU-MIMO service or which UE(s) should receive MU-MIMO service.For instance, the base station could compare the rate of change of RFconditions of a first served UE with the rate of change of RF conditionsof a second served UE. And based at least on a determination that thefirst UE's rate of change of RF conditions is lower (perhaps thresholdlower) than the second UE's rate of change of RF conditions, the basestation could select the first UE rather than the second UE to receiveMU-MIMO service.

FIG. 11 is a flow chart depicting operations that can be carried out inaccordance with this disclosure, to control transmission over an airinterface in a wireless communication system. As shown in FIG. 11, atblock 110, the operations includes a base station serving a plurality ofUEs over the air interface, where each UE of the plurality has arespective rate of change of RF conditions (which could be zero if theUE's RF conditions are unchanging). At block 112, the operations includethe base station selecting at least one of the UEs of the plurality toserve with MU-MIMO over the air interface, with the selecting beingbased at least on the rate of change of RF conditions respectively ofeach selected UE. And at block 114, the operations include, based on theselecting, the base station configuring MU-MIMO service of each selectedUE while serving any non-selected UE (e.g., each other UE) of theplurality of UEs without use of MU-MIMO.

In line with the discussion above, the operations could additionallyinclude determining (e.g., estimating) respectively for each UE theplurality the rate of change of RF conditions of the UE. For instance,the base station could determine the rate of change of RF conditions ofthe UE based on one or more of the metrics noted above.

In addition, as discussed above, the act of selecting at least one ofthe UEs of the plurality to serve with MU-MIMO over the air interfacewith the selecting being based at least on the rate of change of RFconditions respectively of each selected UE could involve, for each UEof the plurality (i) making a determination of whether the rate ofchange of RF conditions of the UE is lower than a predefined thresholdlevel, (ii) if the determination is that the rate of change of RFconditions of the UE is lower than the predefined threshold level, then,based at least on the determination, selecting the UE to serve withMU-MIMO over the air interface, and (iii) if the determination is thatthe rate of change of RF conditions of the UE is not lower than thepredefined level threshold, then, based at least on the determination,not selecting the UE to serve with MU-MIMO over the air interface.

Further, the act of selecting at least one of the UEs of the pluralityto serve with MU-MIMO over the air interface with the selecting beingbased at least on the rate of change of RF conditions respectively ofeach selected UE could involve (i) comparing the rate of change of RFconditions of a first one of the UEs of the plurality with the rate ofchange of RF conditions of a second one of the UEs of the plurality,(ii) based on the comparing, determining that the rate of change of RFconditions of the first UE is lower than the rate of change of RFconditions of the second UE, and (iii) based on the determining,selecting the first UE rather than the second UE to receive MU-MIMOservice over the air interface.

Still additionally, as discussed above, the operations could compriseincluding each selected UE in a MU-MIMO group of UEs based on a furtherdetermination that the selected UE is orthogonal to each other UE of theMU-MIMO group.

And still further, as discussed above, the act of the base stationconfiguring MU-MIMO service of each selected UE could involve the basestation causing air interface transmission between the base station andthe selected UE to occupy same time-frequency air interface resources(e.g., PRBs) as air interface transmission between the base station andanother UE of plurality. Whereas, the act of the base station servingeach other UE of the plurality of UEs without use of MU-MIMO couldinvolve the base station causing air interface transmission between thebase station and each other UE to occupy different time-frequency airinterface resources than air interface transmission between the basestation and any other UE of the plurality.

FIG. 12 is another flow chart depicting operations that can be carriedout in accordance with this disclosure, to control transmission over anair interface in a wireless communication system. As shown in FIG. 12,at block 120, the operations include a base station serving a pluralityof UEs over the air interface, where each UE of the plurality has arespective rate of change of RF conditions. At block 122, the operationsinclude the base station determining, respectively for each UE of theplurality, whether the rate of change of RF conditions of the UE isthreshold low. And at block 124, the operations include the base stationlimiting application of MU-MIMO to UEs of the plurality whose rate ofchange of RF conditions the base station determined to be threshold low.

In line with the discussion above, the operations could additionallyinclude determining respectively for each UE the plurality the rate ofchange of RF conditions of the UE. Here again, for instance, the basestation could determine the UE's rate of change of RF conditions invarious ways as discussed above.

Further, the act of the base station limiting application of MU-MIMO toUEs of the plurality whose rate of change of RF conditions the basestation determined to be threshold low could involve (i) the basestation selecting at least a first UE of the plurality to receiveMU-MIMO service, wherein the selecting is based at least on the rate ofchange of RF conditions of the first UE being at least as low as apredefined threshold level, and (ii) rejecting by the base stationapplication of MU-MIMO service for at least a second UE of theplurality, wherein the rejecting is based at least on the rate of changeof RF conditions of the second UE not being at least as low as thepredefined threshold level.

Configuring MU-MIMO Service of Selected UEs

Through consideration of one or more of the above-discussed factors,among other possible factors, the base station could thus determinewhich of the base station's served UEs to provide with MU-MIMO service.Further, in the course of selecting the UEs to receive MU-MIMO serviceand/or as a separate step, the base station could establish one or moreMU-MIMO groups of UEs to receive MU-MIMO service, based at least on theUEs in each MU-MIMO group being sufficiently orthogonal to each other asdiscussed above.

The base station could then configure MU-MIMO service of the selectedUEs, and particularly of each MU-MIMO group.

For each MU-MIMO group, the base station could engage in signaling witheach member UE to obtain channel estimates in order to facilitatebeamforming and/or precoding of transmissions respectively to each UE.

Further, for each MU-MIMO group, the base station could causeair-interface transmissions between the base station all UEs of theMU-MIMO group to occupy the same time-frequency air interface resources(e.g., the same PRBs and constituent resource elements) as each other.For instance, the base station could transmit respectively to each UE inthe group a DCI message that specifies allocated PRBs that will carrydata to the UE, and the base station could thereby allocate the samePRBs to each UE in the group. In contrast, for any UE that the basestation decides to not provide with MU-MIMO service, the base stationcould schedule transmissions between the base station and the UE tooccur on different time-frequency air interface resources (e.g.,different PRBs) than those that the base station schedules for use withrespect to any other served UE.

Example Base Station Structure

FIG. 13 is a simplified block diagram of an example base station thatcould operate in accordance with the present disclosure. to controltransmission over an air interface in a wireless communication system.

As shown in FIG. 13, the example base station includes a wirelesscommunication interface 1300, a backhaul communication interface 1302,and a controller 1304, which could be integrated together and/orcommunicatively linked by a network, system bus, or other connectionmechanism 1306.

Wireless communication interface 1300 includes a radio 1308, a poweramplifier 1310, and antenna structure 1312. The radio could operate tointerface between encoded baseband signals and RF signals. The poweramplifier could operate to amplify signals for transmission by theantenna structure 1312. And the antenna structure 1312 could comprise aplurality of antennas for communicating over the air interface, wherethe air interface defines physical channel resources for carrying datawirelessly from the base station to a plurality of UEs served by thebase station. As discussed above, the antenna structure could comprisean antenna array, such as a massive-MIMO array for instance.

Backhaul communication interface 1302 could be a network communicationinterface (e.g., an Ethernet network interface port and/or connection)through which the base station can communicate with various othernetwork entities.

And controller 1304, which could comprise a processing unit, datastorage, and program instructions stored in the data storage andexecutable by the processing unit, or could take other forms, could beoperable to cause the base station to carry out various operations asdescribed herein, for scheduling use of the physical channel resourcesto carry data wirelessly from the base station to the UEs

Various features described above could be applied in this context, andvice versa.

Exemplary embodiments have been described above. Those skilled in theart will understand, however, that changes and modifications may be madeto these embodiments without departing from the true scope and spirit ofthe invention.

We claim:
 1. A method for controlling transmission over an air interfacein a wireless communication system, the method comprising: serving, by abase station, a plurality of user equipment devices (UEs) over the airinterface, wherein each UE of the plurality has a respective rate ofchange of radio frequency (RF) conditions for communication between thebase station and the UE; selecting by the base station at least one ofthe UEs of the plurality to serve with Multi-UserMultiple-Input-Multiple-Output (MU-MIMO) over the air interface, theselecting being based at least on the rate of change of RF conditionsrespectively of each selected UE; and based on the selecting,configuring by the base station MU-MIMO service of each selected UE,while serving by the base station each other UE of the plurality of UEswithout use of MU-MIMO.
 2. The method of claim 1, further comprisingdetermining respectively for each UE of the plurality the rate of changeof RF conditions of the UE.
 3. The method of claim 2, whereindetermining the rate of change of RF conditions of the UE is based onone or more metrics selected from the group consisting of channelquality indicator, reference signal receive power, reference signalreceive quality, block error rate, and retransmission rate.
 4. Themethod of claim 1, wherein selecting at least one of the UEs of theplurality to serve with MU-MIMO over the air interface with theselecting being based at least on the rate of change of RF conditionsrespectively of each selected UE comprises, for each UE of theplurality: making a determination of whether the rate of change of RFconditions of the UE is lower than a predefined threshold level; if thedetermination is that the rate of change of RF conditions of the UE islower than the predefined threshold level, then, based at least on thedetermination, selecting the UE to serve with MU-MIMO over the airinterface; and if the determination is that the rate of change of RFconditions of the UE is not lower than the predefined level threshold,then, based at least on the determination, not selecting the UE to servewith MU-MIMO over the air interface.
 5. The method of claim 1, whereinselecting at least one of the UEs of the plurality to serve with MU-MIMOover the air interface with the selecting being based at least on therate of change of RF conditions respectively of each selected UEcomprises: comparing the rate of change of RF conditions of a first oneof the UEs of the plurality with the rate of change of RF conditions ofa second one of the UEs of the plurality; based on the comparing,determining that the rate of change of RF conditions of the first UE islower than the rate of change of RF conditions of the second UE; andbased on the determining, selecting the first UE rather than the secondUE to receive MU-MIMO service over the air interface.
 6. The method ofclaim 1, further comprising including each selected UE in a MU-MIMOgroup of UEs based on a further determination that the selected UE isorthogonal to each other UE of the MU-MIMO group.
 7. The method of claim1, wherein configuring by the base station MU-MIMO service of eachselected UE comprises causing by the base station air interfacetransmission between the base station and the selected UE to occupy sametime-frequency air interface resources as air interface transmissionbetween the base station and another UE of the plurality, and whereinserving by the base station each other UE of the plurality of UEswithout use of MU-MIMO comprises causing by the base station airinterface transmission between the base station and each other UE tooccupy different time-frequency air interface resources than airinterface transmission between the base station and any other UE of theplurality.
 8. A method for controlling transmission over an airinterface in a wireless communication system, the method comprising:serving, by a base station, a plurality of user equipment devices (UEs)over the air interface, wherein each UE of the plurality has arespective rate of change of radio frequency (RF) conditions;determining by the base station, respectively for each UE of theplurality, whether the rate of change of RF conditions of the UE isthreshold low; and limiting by the base station application ofMulti-User Multiple-Input-Multiple-Output (MU-MIMO) to UEs of theplurality whose rate of change of RF conditions the base stationdetermined to be threshold low.
 9. The method of claim 8, furthercomprising determining respectively for each UE of the plurality therate of change of RF conditions of the UE.
 10. The method of claim 9,wherein determining the rate of change of RF conditions of the UE isbased on one or more metrics selected from the group consisting ofchannel quality indicator, reference signal receive power, referencesignal receive quality, block error rate, and retransmission rate. 11.The method of claim 8, wherein limiting by the base station applicationof MU-MIMO to UEs of the plurality whose rate of change of RF conditionsthe base station determined to be threshold low comprises: selecting bythe base station at least a first UE of the plurality to receive MU-MIMOservice, wherein the selecting is based at least on the rate of changeof RF conditions of the first UE being at least as low as a predefinedthreshold level; and rejecting by the base station application ofMU-MIMO service for at least a second UE of the plurality, wherein therejecting is based at least on the rate of change of RF conditions ofthe second UE not being at least as low as the predefined thresholdlevel.
 12. The method of claim 8, further comprising grouping by thebase station UEs of the plurality into a MU-MIMO group based on adetermination that the UEs of the MU-MIMO group are orthogonal to eachother.
 13. A base station operable in a wireless communication system tocontrol transmission over an air interface, the base station comprising:an antenna array comprising a plurality of antennas for communicatingover the air interface, wherein the air interface defines physicalchannel resources for carrying data wirelessly from the base station toa plurality of user equipment devices (UEs) served by the base station,wherein each UE of the plurality has a respective rate of change ofradio frequency (RF) conditions; and a controller for scheduling use ofthe physical channel resources to carry data wirelessly from the basestation to the UEs, wherein the controller is configured to select atleast one of the UEs of the plurality to serve with Multi-UserMultiple-Input-Multiple-Output (MU-MIMO) over the air interface, theselecting being based at least on the rate of change of RF conditionsrespectively of each selected UE, and wherein the controller is furtherconfigured, based on the selecting, to cause the base station to provideMU-MIMO service to each selected UE while the base station serves eachother UE of the plurality of UEs without use of MU-MIMO.
 14. The basestation of claim 13, wherein the controller comprises a processing unit,data storage, and program instructions stored in the data storage andexecutable by the processing unit to carry out the selecting and thecausing.
 15. The base station of claim 13, wherein the controller isfurther configured to determine respectively for each UE the pluralitythe rate of change of RF conditions of the UE.
 16. The base station ofclaim 15, wherein determining the rate of change of RF conditions of theUE is based on one or more metrics selected from the group consisting ofchannel quality indicator, reference signal receive power, referencesignal receive quality, block error rate, and retransmission rate. 17.The base station of claim 13, wherein selecting at least one of the UEsof the plurality to serve with MU-MIMO over the air interface with theselecting being based at least on the rate of change of RF conditionsrespectively of each selected UE comprises, for each UE of theplurality: making a determination of whether the rate of change of RFconditions of the UE is lower than a predefined threshold level; if thedetermination is that the rate of change of RF conditions of the UE islower than the predefined threshold level, then, based at least on thedetermination, selecting the UE to serve with MU-MIMO over the airinterface; and if the determination is that the rate of change of RFconditions of the UE is not lower than the predefined threshold level,then, based at least on the determination, not selecting the UE to servewith MU-MIMO over the air interface.
 18. The base station of claim 13,wherein selecting at least one of the UEs of the plurality to serve withMU-MIMO over the air interface with the selecting being based at leaston the rate of change of RF conditions respectively of each selected UEcomprises: comparing the rate of change of RF conditions of a first oneof the UEs of the plurality with the rate of change of RF conditions ofa second one of the UEs of the plurality; based on the comparing,determining that the rate of change of RF conditions of the first UE islower than the rate of change of RF conditions of the second UE; andbased on the determining, selecting the first UE rather than the secondUE to receive MU-MIMO service over the air interface.
 19. The basestation of claim 13, wherein the controller is further configured toinclude each selected UE in a MU-MIMO group of UEs based on a furtherdetermination that the selected UE is orthogonal to each other UE of theMU-MIMO group.
 20. The base station of claim 13, wherein the antennaarray is a massive-MIMO antenna array.